Multiply $3 \sqrt{7}$ and $5 \sqrt{7}$.

Rationalise the denominator of the following: $\frac{\sqrt{40}}{\sqrt{3}}$

Simplify: $(\frac{1}{27})^{\frac{-2}{3}}$

If $\sqrt{2} = 1.4142$,then evaluate $\sqrt{5} \div \sqrt{10}$ correct to four decimal places.

Express $0.7\overline{39}$ in the form $\frac{p}{q}$,where $p$ and $q$ are integers and $q \neq 0$.

For each question,select the proper option from four options given,to make the statement true: $\sqrt{5} \times \sqrt{5}$ is a / an $\ldots \ldots \ldots$ number.